[emeritus]

No personal experience, but I have a 1940's book on battery charging that covers both NiFe and NiCd batteries. For NiCds, as well as periodic topping up with distilled water to compensate for evaporation, total replacement of the electrolyte was recommended once a year, as indicated by the normal S.G. of the electrolyte dropping from 1.19 when fresh , to 1.16. The deterioration was said to be due to reaction between the Potassium Hydroxide of the electrolyte with atmospheric Carbon Dioxide, producing highly soluble Potassium Bicarbonate.